1. Field of the Invention
The invention relates to a method for the manufacture of vitreous carbon bodies.
2. Description of the Prior Art
For the manufacture of carbon and graphite bodies, milled coke or other solids consisting substantially of carbon are, in general, mixed with a carbon-containing binder. The mixture is then molded and the blank is heated to convert the binder into coke connecting the carbon grains. Accordingly, the bodies are polygranular, as a rule anisotropic and, as a consequence of the binder pyrolysis, more or less porous. There has been no lack of attempts to manufacture more uniform bodies containing substantially only a single phase. It has been proposed, for instance, to harden and carbonize thermosetting plastics, to comminute the product, to mix it with a phenolic resin binder and to process it as usual into a carbon body. In spite of these more extensive procedures, the two-phase structure of the carbon body, characterized by the primary resin coke and the secondary binder coke, remains substantially preserved. A "monogranular" carbon body is obtained from thermosetting plastics which are processed into blanks and, after thermal or catalytic hardening, are carbonized by heating them to 1000.degree. C. The product, which has greater strength than "polygranular" carbon and is substantially impermeable to gases and liquids, is designated as glassy carbon or vitreous carbon because of the glass-like fracture surfaces. In principle, any thermosetting plastic is suitable as the starting material for vitreous carbon, but phenolic resins (British Pat. No. 956,452), furane resins (British Pat. No. 921,236) and mixtures of these substances are predominantly used in liquid or powder form. Liquid resins are cast into molds for making the blanks. Resins in powder form are shaped by die-molding, predominantly hot-pressing, and the resins are hardened by gradually raising the temperature. The term "hardening" is understood to include all condensation and cross-linking reactions which make the resin body unmeltable, so that the bodies can be converted into carbon in a second process step without major changes in shape. For this purpose, the hardened blanks are heated in an inert or reducing atmosphere or in a vacuum to about 800.degree. C. or more. The rate of heating and the maximum heating temperature depend substantially on the size and wall thickness of the blanks and the intended use of the vitreous carbon. Common are rates of about 1 to 5 K./h below about 600.degree. C. and of about 30 K./h above this temperature limit. The maximum heating temperature is advantageously about 1000.degree. C. and about 2800.degree. C. for "graphitized" vitreous carbon.
A weighty disadvantage of the described manufacture of vitreous carbon bodies is the poor formability of thermosetting resins as compared with thermoplastic materials. The forming processes are comparatively expensive and hardly suitable for the manufacture of elongated bodies such as tubes, strips and the like. While it is known to extrude compounds containing granular and fibrous fillers and to produce elongated blanks, this method is less well suited for processing resin types without filler. The fluidity variations in extruding thermosetting resins without filler, which are hardly controllable technically, cause large deviations in dimensions and, above all, lead to a high percentage of rejects due to the formation of cracks in the carbonization stage.